BACKGROUND: Psoriasis is a chronic and recurrent inflammatory skin disease. The inflammatory response represents a fundamental ability of the organism to protect itself from infectious agents and from injury. OBJECTIVES: To evaluate the inflammatory response in mild and in severe psoriasis, to evaluate the endogenous systems counterbalancing the deleterious effects of the inflammation products, and to establish values of prognostic significance. METHODS: The study was performed in a control group (n = 40) and in 60 patients with psoriasis vulgaris, half presenting with mild psoriasis, and the other half with severe psoriasis. We evaluated total and differential leucocyte count; elastase, lactoferrin and lipid peroxidation as markers of neutrophil activation; total plasma antioxidant capacity (TAS), transferrin, ceruloplasmin, alpha(1)-antitrypsin and alpha(2)-macroglobulin as markers of the endogenous antioxidant and antiprotease systems; and fibrinogen, erythrocyte sedimentation rate, C-reactive protein (CRP), haptoglobin, C3 and C4 complement proteins as markers of inflammation. RESULTS: Our data suggested that psoriasis is an inflammatory condition in which neutrophils seem to play a crucial role by contributing to the development of oxidative and proteolytic stress. The worsening of the disease seemed to be linked to the enhancement of the inflammatory response and of the imbalance between neutrophil activation products and their inhibitors. CONCLUSIONS: We propose values for elastase, CRP, elastase/alpha(2)-macroglobulin, elastase/alpha(1)-antitrypsin, thiobarbituric acid/TAS and elastase/neutrophil ratios with prognostic significance for the worsening of psoriasis.